The present invention claims priorities from Japanese Patent Applications No.9-205706 filed Jul. 31, 1997, No.9-210562 filed Aug. 5, 1997 and No.9-273268 filed Sep. 22, 1997, which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an Optical Add-Drop Multiplexer (OADM) capable to of switching a transmission line of a wavelength-multiplexed optical signal without electrically terminating the optical signal and an optical signal monitor utilizing the OADM.
2. Description of the Related Art
As the optical network, a network constructed on the basis of SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy) using the time division multiplexing (TDM) has been used popularly. In a network based on this SONET/SDH, the switching of transmission line is usually performed by terminating an optical signal by a line layer, switching the transmission line and then multiplexing the optical signal again.
In this transmission line switching, in order to maintain the reliability of link connection, reliability evaluation values such as loss of frame (LOF), loss of pointer (LOP), bit error rate (BER), etc., are detected by detecting loss of signal (LOS) as described in, for example, GR-253-CORE (Issue Dec. 1, 1994) describing Bellcore (Bell Communications Research) standard or evaluating section overhead (SOH) in a frame or line overhead (LOH).
Recently, an OADM which is capable of performing the transmission line switching without electrically terminating an optical signal has been proposed and its introduction into a practical system has been studied. Contrary to the transmission line switching in the digital cross-connect system (DCS) which is one of network elements of the SONET is performed through opto-electric (O/E) conversion, a demultiplexer (DEMUX), a switching, a multiplexer (MUX) and an electro-optical (E/O) conversion, the transmission line switching in the OADM can be done by only the switching and the DEMUX and the MUX for the electric signal are unnecessary. Therefore, it is possible in the OADM to reduce amount of hardware per transmission optical signal rate and to reduce the cost and the system size.
In a case where some failure occurs in a network having an OADM, the OADM is required to detect and judge faults such as loss of signal light due to breakage of optical fiber and degradation of signal light quality due to failure of such as optical repeater/amplifier and to perform a recovering operation corresponding to the kind of fault. In a case where a network fault is detected by utilizing a standard frame overhead in the currently used SONET/SDH, there is a necessity occurs in the OADM of terminating a section or line.
When a fault occurs in the line including the OADM, in order to notify an end user of the line the occurrence of the fault, read/write processing of an alarm indication signal (AIS) from the frame overhead becomes necessary in the OADM. In an OADM dealing with a high density multiplexed optical signal, however, there are problems that it is necessary to not only provide a large scale hardware in order to obtain the AIS from the frame overhead of the transmitted optical signal, but also electrically terminate the multiplexed optical signal, read the frame overhead and multiplex the optical signals again in order to merely read a fault monitor information within the network.
Further, in order to introduce the OADM into the currently existing optical network, it is necessary to deal with the optical signal separately from the time-multiplex orientated SONET/SDH layer, electrically process it synchronously in time and define a wavelength/space multiplexing orientated layer which does not obtain an information from that optical signal or does not add information thereto as an optical layer to thereby make an automatic protection system (APS) which is network fault recovery means supplied by the SONET/SDH non conflict with a protection system in the optical layer newly supplied by the OADM.
This is because, even if the SONET/SDH be disappeared in an ideal construction of future optical network, the resources of the SONET/SDH must be used for at least the present since the SONET/SDH is currently in world wide use and, since it is difficult in view of cost to immediately change from the APS to the protection system by the OADM, it is necessary to exist APS together the protection in the optical layer using the OADM for at least the present.
Considering the introduction of the OADM into a practical network, the ability of the OADM can not be fully used regardless of the high potential thereof which makes the reduction amount of hardware per transmitted optical signal possible, due to the above mentioned problems. In order to use the ability of the OADM sufficiently, it is indispensable to develop a system for monitoring a network fault and recovering the fault suitably for a network dealing with high density multiplexed signal and a hardware therefor while maintaining the compatibility with respect to the SONET/SDH.
An object of the present invention is to provide an OADM capable of dropping/adding a signal light having arbitrary wavelength within a node and to reduce cost and size of the OADM node by defining a fault detection signal of an optical signal cut-off in a wavelength/space multiplexing oriented optical layer which does not conflict with a transmitter of SONET/SDH or other framing format.
According to a first aspect of the present invention, the OADM node comprises an optical wavelength division coupler for wavelength-dividing a wavelength-multiplexed light In input from one optical transmission line to optical signals having respective wavelengths, a plurality of light receivers each for receiving different one of the respective wavelength optical signals from the optical wavelength division coupler, a plurality of optical gate switches each for on-off controlling different one of the respective wavelength signals, a plurality of light transmitters provided correspondingly to the plurality of the light receivers for-transmitting the respective wavelength optical signals, respectively, and an optical wavelength multiplex coupler connected to outputs of the optical gate switches and outputs of the light transmitters for wavelength-multiplexing the respective wavelength optical signals and sending the wavelength-multiplexed optical signal to another optical transmission line.
The optical gate switch should have a low cross-talk and low insertion loss characteristics. For example, a semiconductor optical switch using a semiconductor optical amplifier may be suitable for the optical gate switch.
However, when gain of such semiconductor optical amplifier is small, S/N ratio is degraded due to spontaneously emitted light noise when a signal light is input to the optical amplifier. Therefore, it is necessary to increase the gain. However, since, when an output of the semiconductor optical amplifier is increased, inverted population density of carrier is reduced, gain is saturated, resulting in an output saturation. Under the condition of such output saturation, the signal waveform is distorted, causing receiving sensitivity to be degraded. Further, since, when such semiconductor optical switch is moduled, optical coupling loss of a semiconductor chip and optical fiber is several dB, the saturated output power becomes small. Since a practical difference between output power limit and S/N degradation is as narrow as several dB, the freedom of design of optical level in constructing a system is substantially restricted. Under such conditions, a severe optical level regulation is necessary, so that the number of monitor items for light intensity is increased, which restricts the reduction of scale of a node. For these reasons, a realization of an optical switch having low cross-talk and large saturation output power is expected.
According to the present invention, an impurity doped optical fiber is used and a signal light input to the input side optical transmission line is on-off controlled by controlling an intensity of a pumping light and an intensity of the signal light output to the output side optical transmission line is controlled by controlling the intensity of the pumping light during an on state of the input signal light. Further, a combination of such impurity doped optical fiber amplifier and a semiconductor optical amplifier may be utilized as the optical switch.
In the latter case, the following various constructions may be realized:
(a) an optical filter which allows only an input signal light to pass through is inserted into an output side optical transmission line;
(b) a reflection mirror which reflects an output signal light back to the impurity doped fiber is arranged on an output end of the output side optical transmission line and an optical coupler for dropping (branching) the signal light output from the impurity doped fiber is inserted into the input side optical transmission line;
(c) the output end of the output side optical transmission line is connected to the optical coupler inserted into the input side optical transmission line so that the signal light output from the output side optical transmission line is transmitted through the input side optical transmission line in a direction opposite to the input signal light;
(d) an optical coupler for connecting the pumping light output from the pumping light in source to the impurity doped fiber is provided on the input side of the impurity doped fiber;
(e) an optical coupler for connecting the pumping light output from the pumping light source to the impurity doped fiber is provided on the output side of the impurity doped fiber;
(f) optical isolators are inserted into arbitrary positions of the input side and output side optical transmission lines, respectively;
(g) a plurality of pumping light sources are provided and pumping lights from the respective pumping light sources are wavelength-multiplexed and input to the impurity doped fiber.
In such optical switch, it is possible to realize an optical switch having the low cross-talk which can be freely set by on-off controlling the pumping power in the pumping light source and the insertion loss of the switch which is freely set by controlling the pumping power. That is, it is possible to obtain an optical switch which has low cross-talk and high output power characteristics and an input/output construction thereof can be is freely set according to a system demand.
According to a second aspect of the present invention, an OADM comprises an optical circulator for receiving a wavelength-multiplexed light from one optical transmission line and supplying a wavelength multiplexed light to the other optical transmission line, an optical wavelength division multiplex coupler connected to the circulator for, on one hand, wavelength-dividing the wavelength-multiplexed light input from the input of the circulator to a plurality of wavelength lights having different wavelengths and for, on the other hand, wavelength multiplexing the plurality of wavelength lights having different wavelengths and supplying it to the optical circulator, first optical couplers connected to the optical wavelength division multiplex coupler, for receiving/sending a plurality of wavelength lights having different wavelengths with respect to the optical wavelength division multiplex coupler, respectively, light receivers connected to the respective first optical couplers for receiving optical outputs of the first optical couplers, light transmitters provided correspondingly to the light receivers for supplying the plurality of wavelength lights having different wavelengths constructing the wavelength multiplexed light optical gate switches provided correspondingly to the wavelength-divided lights, each optical gate switch for on-off controlling a light passing therethrough, second optical couplers provided correspondingly to the respective wavelength-divided lights and connected to the respective light transmitters for receiving lights from the light transmitters, respectively, the second optical couplers being connected, on one sides thereof, to the optical inputs/outputs of the first optical couplers, respectively, and, on the other sides thereof, to one sides of the optical gate switches to input/output wavelength lights with respect thereto, respectively, and light reflection mirrors provided correspondingly to the respective wavelength-divided lights and connected to the other sides of the optical gate switches, respectively. The wavelength-multiplexed light output from the light input/output of the optical circulator and received by the optical wavelength division multiplex coupler is output from the optical wavelength division multiplex coupler to the optical transmission line through the light output of the optical circulator.
According to a third aspect of the present invention, an OADM comprises an optical wavelength division multiplex coupler for wavelength-dividing a wavelength-multiplexed light input from an optical transmission line on one hand and wavelength-multiplexing the wavelength-divided lights and supplying a wavelength-multiplexed light to the optical transmission line on the other hand, first optical couplers connected to the optical wavelength division multiplex coupler, the first optical couplers being adapted to input/output the wavelength lights having different wavelengths with respect to the optical wavelength division multiplex coupler on one hand and to drop portions of the wavelength lights having different wavelengths, respectively, on the other hand, light receivers connected to the respective first optical couplers for receiving the dropped optical outputs of the first optical couplers, respectively, light transmitters provided correspondingly to the respective light receivers for supplying the plurality of wavelength lights having different wavelengths constructing the wavelength-multiplexed light, respectively, optical gate switches provided correspondingly to the respective wavelength-divided lights, each optical gate switch being adapted to on-off control a light passing therethrough, optical isolators provided correspondingly to the respective wavelength-divided lights and adapted to pass only corresponding light outputs of the optical gate switches, respectively, second optical couplers provided correspondingly to the respective wavelength-divided lights, the second optical couplers being connected to the light inputs/outputs of the first optical couplers, the second optical couplers being adapted to drop portions of the respective wavelength-divided lights and to receive lights from the respective light transmitters and third optical couplers provided correspondingly to the respective wavelength-divided lights, the third optical couplers being connected to the light inputs/outputs of the second optical couplers, the second optical couplers being adapted to receive the light outputs of the respective optical isolators and output wavelength lights having different wavelengths from the wavelengths of the output lights of the optical isolators to the optical gate switches. One of the respective wavelength-divided lights is passed through a path including one of the first optical couplers, one of the second optical couplers, one of the third optical couplers, one of the optical gate switches and one of the optical isolators, corresponding to one wavelength light, and returned to the optical wavelength division multiplex coupler through a path including another of the third optical couplers and another of the second optical coupler, corresponding to another wavelength light having wavelength adjacent to that of the one wavelength light and the optical wavelength division multiplex coupler wavelength-multiplexes the returned lights.
According to another aspect of the present invention, an OADM comprises control means for selecting one of pass, drop and add of an optical signal by selectively switching on/off states of the optical gates, presence/absence of optical signal input to the light receivers and presence/absence of optical signals output from the light transmitters and optical signal fault monitor means having functions of monitoring an optical signal cut-off fault by either detecting the optical loss of signal (OLOS) by determining presence or absence of a signal light on a basis of calculation of an optical S/N ratio from a ratio of intensity of light having wavelength within a wavelength range of a certain width including at least the signal light to intensity of spontaneously emitted light having wavelength within a different wavelength range of substantially the same width as the predetermined width or determining presence or absence of the signal light from intensity of light within a wavelength range of a certain width including the signal light, or detecting the optical loss of wavelength (OLOW) by monitoring wavelength deviation of the signal light from intensity of light within the wavelength range and a function of calculating an optical S/N ratio from a ratio of intensity of light within a wavelength range of a predetermined width including at least the signal light to intensity of spontaneously emitted light having wavelength within a different wavelength range of substantially the same width as the predetermined width and detecting the optical signal degrade (OSD) when the optical S/N ratio becomes smaller than a predetermined threshold value, for detecting at least one of the OLOS, the OLOW and the OSD as fault detection signals, wherein, when the optical signal fault monitor means detects at least one of the fault detection signals, the control means controls the gates to cut off signal lights from an output corresponding to the detected input to convert the detected fault detection signal into an optical alarm indication signal (AIS-O).
It is possible to monitor an optical signal fault by utilizing the above mentioned OADM. That is, an optical network provided with these OADM""s, optical repeaters/amplifiers and line terminators for terminating optical signal and processing an optical signal obtained by wavelength-multiplexing light signals each on an optical fiber comprises the above mentioned optical signal fault monitor means as an optical layer monitor,
An optical signal fault monitor system according to another aspect of the present invention has a function of converting a fault signal into the optical alarm indication signal (AIS-O) in an optical layer by cutting off the signal light from an output correspondingly to the fault detection signal detected by the control means when the OADM detects at least one of the fault detection signals. With this AIS-O conversion function, the signal processing is simplified since an electric signal obtained by electrically terminating an optical signal wavelength multiplexed with at least one wavelength in time synchronism contains no signal information and there is no case where any new information is added.
In optical signal fault monitor system according to another aspect of the present invention, an optical network for processing an optical signal wavelength-multiplexed with at least one wavelength in each optical fiber comprises OADM""s and optical repeaters/amplifiers provided with at least the above mentioned optical layer monitor, respectively, and a transmission device connected to the OADM""s, the optical repeaters/amplifiers and the optical layer, the optical layer monitor monitors an optical signal cut-off fault in the network by at least one of OLOS and AIS-O in the optical layer.
In an optical signal fault monitor system according to a further aspect of the present invention, in which the optical signal fault of the optical network is monitored by utilizing one of the OADM and the optical repeater/amplifier, a network fault is noticed to a transmission device connecting the optical layer to other layer than the optical layer by converting the OLOS of the network fault in the optical layer into the AIS-O.
For recovery of fault, when a fault of the optical network is monitored by using the OLOS and the AIS-O in the optical layer instead of a signal for informing an occurrence of fault of a transmission device connected to the optical layer by utilizing one of the OADM and the optical repeater/amplifier and when a signal reproducing repeater is arranged on an upstream side of a detected location of the network fault of the OADM in the optical network, the recovery is performed by arranging at least one line terminator connected to the optical layer between the OADM and the signal reproducing repeater and sending a monitor signal issued by the signal reproducing repeater to the line terminator so that an occurrence of the network fault is noticed from the signal reproducing repeater to the line terminator when the signal reproducing repeater detects the network fault, by an optical path switching performed by the optical add-drop multiplexer in the optical layer according to a notice of the network fault to at least one of the optical repeater/amplifier on the downstream side of the signal reproducing repeater and the OADM or by a notice of network fault performed by a transmission line connected to the optical layer from the optical layer to the transmission line by converting the OLOS of the network fault in the optical layer into the AIS-O. Further, a network management information is assigned to a wavelength range different from a main signal wavelength range to be processed by the OADM to perform a network information communication between the nodes.
With these means, it is possible to the fault monitoring, the fault notice and the network information communication independently from framing format of the transmission device connected to the optical layer.
Further, it is possible, in an optical network including OADM""s and optical repeaters/amplifiers each provided with the optical layer monitor and a line terminator of the SONET/SDH standard in which an optical signal wavelength-multiplexed with at least one wavelength in each optical fiber is processed and when a network fault is detected in the section layer of the SONET/SDH, to monitor the optical signal cut off fault in the network by at least one of the OLOS and the AIS-O in the optical layer instead of an AID-L in the SONET/SDH layer output again to the line terminator to notice the network fault to the line terminator by reading in the AIS-L assigned to a line overhead to notify the occurrence of the network fault to the line terminator on the downstream side of the location at which the network fault is detected.
When a network fault in the optical layer occurs in the optical network utilizing one of the OADM and the optical repeater/amplifier, it is possible to notice the network fault from the optical layer to the SONET/SDH layer by converting the OLOS into the AIS-O to intentionally detect the LOS by the line terminator.
In an optical network in which a network fault is monitored by using one of the OADM and the optical repeater/amplifier and using at least one of OLOS and AIS-O in the optical layer instead of the AIS-L in the SONET/SDH layer and the signal reproducing repeater of the SONET/SDH standard is arranged on the upstream side of the location of the OADM at which the network fault is detected, it is possible to recovery the network fault by arranging at least one of the line terminator of the SONET/SDH standard between the OADM and the signal reproducing repeater and activating an automatic protection system (APS) of the SONET/SDH standard by sending the AIS-L issued by the signal reproducing repeater to the line terminator so that the occurrence of the network fault is noticed from the signal reproducing repeater to the line terminator of the SONET/SDH standard on the downstream side when the network fault is detected by the signal reproducing repeater.
As another fault recovery function in the case where the signal reproducing repeater is arranged on the upstream side of the location of the OADM network at which the network fault is detected, the network fault is recovered by at least one of a switching of at least one of optical paths performed by the OADM in the optical layer according to a notice of the network fault to at least one of said optical repeater/amplifier and the OADM on the downstream side of said signal reproducing repeater and an APS of the SONET/SDH, which is performed by a notice of the network fault from the optical layer to the SONET/SDH layer by converting the OLOS of the network fault in said optical layer into the AID-I to notify the line terminator the detection of the LOS.
Further, the network information communication between the nodes in the optical network for monitoring the network fault by utilizing one of the OADM and the optical repeater/amplifier and using at least one of OLOS and AIS-O in the optical layer instead of the AIS-L in the SONET/SDH layer can be performed between the OADM""s and between the OADM and the line terminator by assigning a network management information to a wavelength range different from a main signal wavelength range.
In a ring network having a plurality of OADM""s each provided with an optical layer monitor connected in a ring by at least one transmission line, it is possible to recover a fault by detecting the fault by using at least one of OLOS and AIS-O and by at least one of a switching to a protect transmission line and a reconfiguration of network.
In these constructions, the fault is monitored by wavelength dividing a wavelength multiplexed light input to the OADM. In view of utilization of the OADM, however, a light intensity monitor which excludes spontaneously emitted optical noise component generated by the optical gate switch having the functions of detecting wavelength deviation of the signal newly added in the OADM and amplifying it is required. However, it is not sufficient to merely measure the light intensity and it is necessary to drop a portion of the wavelength multiplexed light output from the OADM, wavelength-divide the dropped portion and measure the light intensity of the respective wavelength divided lights.
In general, in order to monitor signal lights having different wavelengths, it is necessary to provide band-pass filters each allowing a light having specific wavelength, which causes the size and cost of the device to be increased. In view of this, the signal lights having respective wavelengths are derived from the wavelength-multiplexed light by utilizing the WDM coupler which is necessarily provided in the OADM for wavelength-multiplexing signal lights. In order to perform a monitor of such wavelength-multiplexed light, it is preferable to utilize an optical wavelength division multiplex coupler having two or more input ports and two or more output ports, and having a function of wavelength multiplexing signal lights input from a plurality of optical transmission lines, a function of wavelength-dividing a wavelength multiplexed light input from an optical transmission line to wavelength lights having different wavelengths and an optical router function of providing a regular correlation between the optical input ports and the optical output ports for every wavelength to wavelength-multiplex signal lights input from a plurality of optical transmission lines, to drop a portion of a wavelength-multiplexed light output to an optical transmission line, to input the dropped portion to the optical wavelength division multiplex coupler again and to monitor lights output therefrom.
With the latter construction, the wavelength-multiplexed light output from the optical wavelength division multiplex coupler is branched by an optical coupler to a transmission light to be transmitted to a succeeding node and a monitor light to be monitored. The monitor light is input the optical wavelength division multiplex coupler again. The re-divided monitor lights are input to the respective optical transmission lines in the nodes in a direction opposite to the direction of the transmission light and the monitor light is output to the light monitor by an optical coupler provided in the transmission line. Since the monitor light is passed through the optical wavelength division multiplex coupler having a narrow pass band, it is possible to monitor wavelength deviation and to measure a level of light containing no spontaneously emitted optical noise component. That is, since the system of the present invention does not require band-pass filters or the optical wavelength division coupler for the optical monitor, it is possible to realize an optical monitor of low cost.